The present invention relates to internal-combustion engines of the type comprising an electronically controlled hydraulic system for actuation of the inlet and/or exhaust valves.
The present applicant has already proposed, in the U.S. Pat. No. 6,237,551, an engine comprising:
at least one inlet valve and at least one exhaust valve for each cylinder, each provided with respective elastic-return means, which push the valve towards a closed position, for controlling respective induction and exhaust ducts;
at least one camshaft for actuating each inlet valve and exhaust valve of the cylinders of the engine by means of respective tappets, each inlet valve and exhaust valve being controlled by a respective cam of said camshaft,
in which each of said tappets controls the respective inlet valve and exhaust valve against the action of said elastic-return means by the interposition of hydraulic means, which include a pressurized fluid chamber,
the pressurized fluid chamber associated to each inlet valve or exhaust valve being designed for being connected by means of a solenoid valve to an exhaust duct for the purpose of uncoupling the valve from the respective tappet and causing rapid closing of the valve as a result of the respective elastic-return means; and
electronic control means for controlling each solenoid valve for varying the time and stroke of opening of the respective inlet valve or exhaust valve according to one or more operating parameters of the engine.
In engines of the type specified above, movement of the valve depends not only upon the driving electrical command generated by the aforesaid electronic control means, but also upon the viscosity of the hydraulic fluid present in the actuation system. Said viscosity is subject to variations as the operating temperature varies.
The present applicant has already proposed in the past solutions that take into account the above problem, for example by intervening on the hydraulic braking device provided in engines of the type described above, the aim being to slow down (brake) the movement of closing of the valve as the valve approaches its completely closed position. The hydraulic braking device is necessary for preventing an excessively violent impact of the valve against its seat but must be excluded when the viscosity of the hydraulic fluid becomes too high, for example in conditions of cold starting at a very low ambient temperature, in so far as closing of the valve would be rendered too slow.
The purpose of the present invention is to provide a radical and more general solution to any problem that might arise during operation of the engine on account of the variations of viscosity of the hydraulic fluid used in the hydraulic system for variable control of the valves.
The above purpose is achieved, according to the invention, mainly by providing at least one temperature sensor for detecting the temperature of the hydraulic fluid in the proximity of the aforesaid hydraulic means for actuation of the valves and by the fact that the aforesaid electronic control means are programmed for estimating the viscosity of the hydraulic fluid according to the signal at output from the aforesaid temperature sensor and for controlling each of the aforesaid solenoid valves of the system for variable control of the engine valves according to the estimated value of viscosity.
Preferably, the electronic control means can be programmed so as to control the solenoid valves according to a pre-set reference criterion corresponding to pre-set reference operating conditions and to correct said criterion according to the estimated difference between the reference operating conditions and actual operating conditions.
The aforesaid pre-set reference operating conditions typically correspond in the first place to a range of values of fluid viscosity. However, the system may be further refined, by providing also a fluid-pressure sensor with the purpose of determining the presence of the fluid in the hydraulic circuit and evaluating the operating conditions not only upon the basis of the signal emitted by the temperature sensor but also on the basis of the signal emitted by the pressure sensor.